1. Field of Invention
This invention relates to an input protecting mechanism for a measuring device; and, more particularly, to such a mechanism which will prevent erroneous input which might otherwise follow erroneous insertion of a plug of a test lead.
2. Description of the Prior Art
A conventional input protecting mechanism for a measuring device is disclosed, for example, in Japan UM Publication 87/39,333. The mechanism in 39,333 aims to prevent a tester, in which a high voltage measuring input terminal is disposed independently of a low voltage measuring input terminal, from having the plug of a test lead inserted into the low voltage measuring input terminal when a high voltage is to be measured. The test lead plug is allowed to be inserted only into the high voltage measuring input terminal by shutting the low voltage measuring input terminal automatically with a shutter in response to the selection of a range when the range select switch is switched to a high voltage measuring range.
With such a conventional device, it is possible to prevent the low voltage measuring input terminal from being supplied with an excessive input. However, the high voltage measuring input terminal is open at all times, and the low voltage measuring input terminal is also opened in case the range select switch is switched to a range other than the high voltage measuring range. Thus, disadvantageously, when the range select switch is switched to a range other than the high voltage measuring range, the plug of the test lead for measuring high voltage might erroneously be inserted into the low voltage measuring input terminal to cause damage to the internal circuitry of the tester by the supply of excessive input.